Add admin LLM model setting; rename Pi to LLM in the UI; drop table Review column

- New server/llm.js consolidates ModelRuntime + model selection: admin
  setting (app_settings.llm_model) > LLM_MODEL/PREFILL_MODEL env > first
  available; used by both prefill and roast evaluation
- GET/PUT /api/admin/llm lists configured models and stores the choice
  (validated against the list; empty = auto; audited); admin page gains
  an LLM section with a model picker
- All user-facing 'Pi agent' wording is now 'LLM'; no_model error message
  no longer references the pi CLI
- /roasts table: Review column removed (review lives in the detail view)

Co-Authored-By: Claude Fable 5 <[email protected]>

README.md

@@ -59,7 +59,10 @@ The planner is responsive and caches its app shell for offline use after the fir
 
 Prefill requires a model configured via the `pi` CLI (`~/.pi/agent/{models,auth}.json`) —
 without one, `/api/prefill` returns `503 no_model` rather than crashing. Everything else
-(the form, ledger, curve, `.alog` upload) works with no model configured.
+(the form, ledger, curve, `.alog` upload) works with no model configured. Which configured
+model runs prefill and roast reviews is chosen in the admin page's **LLM** section
+(`app_settings.llm_model`, "Auto" = first available); the UI refers to the model backend
+simply as "the LLM".
 
 ## Layout